CEDAR RAPIDS, Iowa — Bernie Sanders just can’t bring himself to do it.

His aides, surrogates and supporters are letting it rip in the final days before Monday’s Iowa caucuses. But even as caucus eve polling suggests Sanders trailing slightly behind Hillary Clinton, the senator himself continues to shy away from the kind of hard-hitting, tear-off-the-bark criticism that might be required to finally make clear the contrast between them.

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It’s not that Sanders won’t throw a punch. His campaign just released its toughest ad yet, zeroing in on Goldman Sachs after Sanders spent weeks calling out Clinton for her ties to the bank. Campaigning before 800 supporters in Burlington, Iowa, on Thursday, he also ticked off a series of positions where Clinton has changed her stance – the Defense of Marriage Act, the Trans-Pacific Partnership, the Keystone pipeline.

On Sunday, he finally bit on the Clinton email controversy he so famously brushed off early in the campaign.

“I think this is a very serious issue,” Sanders said on CNN. “I think there is a legal process right now taking place.”

Yet missing from the Sanders critique is a campaign trail staple: the direct, no-holds-barred attack on your opponent. There was no mention of Clinton at all in the Goldman Sachs ad – viewers were left to come to their own conclusions. On Saturday in Charles City, the Vermont senator was content to merely note he has “very strong disagreements with Secretary Clinton on a number of issues,” citing policies on the Iraq War, Social Security, paid family and medical leave, pipeline approvals, trade deals, and accepting help from a super PAC.

The atmosphere is a far cry from 2008, when Clinton and Barack Obama regularly lit into each other as the campaign tightened, fully aware of the campaign imperative to go negative or go home. At one event, Clinton accused Obama of using “tactics right out of Karl Rove’s playbook.” In Pennsylvania, Obama decried her “slash-and-burn politics.” Each had formulaic “shame on you” moments that drove the news cycle.

Sanders’ high-profile surrogates haven’t shown the same reserve as the senator. Two celebrities who regularly introduce him, actress Susan Sarandon and professor Cornel West, set a scalding tone this week.

“We will not be led astray by our sister Hillary Clinton, we know the difference between a genuine, small-d democrat, and not a capital-D Wall Street Democrat,” bellowed West in Davenport on Friday, to a chorus of boos against Clinton that threatened to drown him out.

“I know Hillary Clinton likes to say she’s against the establishment, but I was born at night, not last night,” he followed up the next morning in Manchester.

Sarandon was even harsher — referring to Clinton as “the machine” that Obama faced in 2008 — when she introduced Sanders at a rally in Mason City on Wednesday. The next day, in liberal Fairfield, she continued to fire away. “I’ve gotten a lot of flak for not just voting genitalia and supporting the woman in the campaign,” she said to cheers.

Sanders’ chief strategist Tad Devine insisted that the surrogate introductions aren’t designed to do the dirty work the candidate himself is often unwilling to do.

“Yeah, [Sarandon] does say some things about Bernie that someone may say that’s a slight to Hillary. I get that, but since the beginning of the campaign, when our ad said ‘An Honest Leader,’ people said it was a slight against [Clinton]. What are we supposed to do?” an exasperated Devine asked. “We don’t tell surrogates what to say. The first time I saw Sarandon’s speech was when she delivered it."

The Clinton campaign scoffs at the notion that Sanders is holding anything back. Chief strategist and pollster Joel Benenson has called it “the most negative” Democratic primary campaign ever – and Hillary Clinton all but seconded his view on Saturday.

“Joel has a lot of experience so I’m not going to disagree with him,” she told NBC News.

It’s true that even if Sanders himself hasn’t slammed the pedal to the floor, the tone of the race is increasingly bitter. His raucous crowds now regularly boo loudly if Clinton is named or alluded to in his stump speech. Iowans have reported seeing anti-Clinton materials spread by Sanders fans at her rallies. On Saturday, the campaigns spent the day publicly bickering over the Democratic debate schedule.

As a result, the Clinton team is working overtime to frame Sanders as conventional politician — using conventional tactics – and no political revolutionary. Devine insists it won’t – and can’t – work, pointing to Sanders’ widely-hailed new campaign ads.

“That’s delusional, turn on a TV set. Bernie Sanders is talking about the American horizon, where he wants to lead America, Simon and Garfunkel, endorsements,” he said. “Yeah, we’re saying Goldman Sachs shouldn’t be trying to buy elections, but 85 percent of what we’re saying is super, super positive. And Bernie is out there talking about what he wants to do in the future. Big ideas."

Sanders, whose reluctance to attack is at least in part due to the fact that he’s never really had to in his congressional campaigns, continues to profess his intention to run a high-minded, issue-oriented campaign. He bristles at the idea that he’s done anything but that — and frequently laments the media's interest in having him attack Clinton.

And like Clinton, Sanders sees himself as the victim of unsubstantiated attacks and distortions of his record.

"I am disappointed by the tone of her campaign. She is talking to the people of Iowa saying Bernie Sanders wants to dismantle health care," said a clearly angry Sanders Saturday, to raucous boos from a Cedar Rapids crowd that appeared infuriated by his unflattering description of Clinton's tactics.

"I've been fighting for universal healthcare my entire life! She has run a TV ad suggesting I am attacking I am attacking Planned Parenthood," a yelling Sanders continued. "I have a 100 percent record with Planned Parenthood. I'm not attacking Planned Parenthood, I want to expand Planned Parenthood. The same ad, she says I'm protecting the gun lobby. Some protection. I've got a D- rating from the NRA!"